The extreme and long-lasting marine heatwave in the northeastern Bay of Bengal during 2020 summer

Abstract

The Bay of Bengal is a hotspot for intense marine heatwaves (MHWs) with adverse impacts on local coral ecosystems, yet extreme MHWs in this region remain largely unexplored. In summer 2020, the most extreme and longest-lasting MHW (MHW-20) on record occurred in the northeastern Bay of Bengal (NEB), which persisted for 94 days with a maximum intensity of 1.8°C. The anomalous warming developed locally in central NEB from 20 June to 29 July 2020, and then weakened while extending westward until 21 September 2020. Here, we examine the local and remote physical processes during the evolution of the MHW-20, and perform a heat budget analysis for the mixed layer. We find that the onset of the extreme event was primarily driven by increased shortwave radiation under clear-sky conditions during the break phase of the strongest Monsoon Intraseasonal Oscillation (MISO) event of 2020 and surface convergence from an anticyclonic oceanic gyre. Oceanic downwelling Rossby waves associated with the extreme positive 2019 Indian Ocean Dipole event also played an important role in the generation and persistence of the MHW-20. The decay of the MHW-20 was mainly controlled by enhanced latent heat release due to intensified wind during the MISO active phase, and reduced heat advection due to weakened surface convergence linked to the oceanic gyre and planetary waves westward mitigation out of the NEB region. These findings may improve our knowledge of physical drivers of extreme MHWs that have important implications for the marine environment and life.